Semiconductor nanocrystal heterostructures

ABSTRACT

A semiconductor nanocrystal heterostructure has a core of a first semiconductor material surrounded by an overcoating of a second semiconductor material. Upon excitation, one carrier can be substantially confined to the core and the other carrier can be substantially confined to the overcoating.

CLAIM OF PRIORITY

[0001] This application claims priority to U.S. Patent ApplicationSerial No. 60/402,726, filed on Aug. 13, 2002, the entire contents ofwhich are hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] The U.S. Government may have certain rights in this inventionpursuant to Contract No. N00014-01-1-0787 awarded by the Office of NavalResearch.

TECHNICAL FIELD

[0003] The invention relates to semiconductor nanocrystalheterostructures.

BACKGROUND

[0004] Semiconductor nanocrystals have been a subject of great interest,promising extensive applications including display devices, informationstorage, biological tagging materials, photovoltaics, sensors andcatalysts. Nanocrystals having small diameters can have propertiesintermediate between molecular and bulk forms of matter. For example,nanocrystals based on semiconductor materials having small diameters canexhibit quantum confinement of both the electron and hole in all threedimensions, which leads to an increase in the effective band gap of thematerial with decreasing crystallite size. Consequently, both theoptical absorption and emission of nanocrystals shift to the blue (i.e.,to higher energies) as the size of the crystallites decreases.

[0005] The quantum efficiency of emission from nanocrystals having acore of a first semiconductor material can be enhanced by applying anovercoating of a second semiconductor material such that the conductionband of the second semiconductor material is of higher energy than thatof the first semiconductor material, and the valence band of the secondsemiconductor material is of lower energy than that of the firstsemiconductor material. As a result, both carriers, i.e., electrons andholes, are confined in the core of the nanocrystal.

SUMMARY

[0006] In general, a semiconductor nanocrystal heterostructure has acore of a first semiconductor material surrounded by an overcoating of asecond semiconductor material. The first semiconductor material andsecond semiconductor material are selected so that, upon excitation, onecarrier is substantially confined to the core and the other carrier issubstantially confined to the overcoating. In one example, theconduction band of the first semiconductor material is at higher energythan the conduction band of the second semiconductor material and thevalence band of the first semiconductor material is at higher energythan the valence band of the second semiconductor material. In anotherexample, the conduction band of the first semiconductor material is atlower energy than the conduction band of the second semiconductormaterial and the valence band of the first semiconductor material is atlower energy than the valence band of the second semiconductor material.These band alignments make spatial separation of carriers, i.e. the holeand the electron, energetically favorable upon excitation. Thesestructures are type II heterostructures. In contrast, the configurationsin which the conduction band of the second semiconductor material is ofhigher energy than that of the first semiconductor material, and thevalence band of the second semiconductor material is of lower energythan that of the first semiconductor material are type Iheterostructures. Type I heterostructure nanocrystals favor confinementof both the hole and the electron in the core. The language of type Iand type II is borrowed from the quantum well literature where suchstructures have been extensively studied.

[0007] Nanocrystals having type II heterostructures have advantageousproperties that result of the spatial separation of carriers. In somenanocrystals having type II heterostructures the effective band gap, asmeasured by the difference in the energy of emission and energy of thelowest absorption features, can be smaller than the band gap of eitherof the two semiconductors making up the structure. By selectingparticular first semiconductor materials and second semiconductormaterials, and core diameters and overcoating thicknesses, nanocrystalshaving type II heterostructures can have emission wavelengths, such asinfrared wavelengths, previously unavailable with the semiconductor ofthe nanocrystal core in previous structures. In addition, the separationof charges in the lowest excited states of nanocrystals having type IIheterostructures can make these materials more efficient in photovoltaicor photoconduction devices where the nanocrystals are chromophores andone of the carriers needs to be transported away from the excitationsite prior to recombination.

[0008] Advantageously, a wide variety of nanocrystals having type IIheterostructures can be prepared using colloidal synthesis. Colloidalsynthesis allows nanocrystals to be prepared with controllabledispersibility imparted from coordinating agents, such as ligands, andare prepared in the absence of wetting layers commonly employed innanocrystals having type II heterostructures prepared by molecular beamepitaxy.

[0009] In one aspect, a method of preparing a coated nanocrystalincludes introducing a core nanocrystal including a first semiconductormaterial into an overcoating reaction mixture and overcoating a secondsemiconductor material on the core nanocrystal, wherein the firstsemiconductor material and the second semiconductor material areselected so that, upon excitation, one carrier is substantially confinedto the core and the other carrier is substantially confined to theovercoating. In another aspect, a coated nanocrystal includes a corenanocrystal including a first semiconductor material, an overcoatingincluding a second semiconductor material on the core nanocrystal, andan organic layer on a surface of the coated nanocrystal, wherein thefirst semiconductor material and the second semiconductor material areselected so that, upon excitation, one carrier is substantially confinedto the core and the other carrier is substantially confined to theovercoating. In another aspect, a population of coated nanocrystalsincludes a plurality of coated nanocrystals, each coated nanocrystalincluding a core nanocrystal and an overcoating on each corenanocrystal, wherein each core includes a first semiconductor materialand each overcoating includes a second semiconductor material, theplurality of core nanocrystals forming a population of nanocrystals,wherein the first semiconductor material and the second semiconductormaterial are selected so that, upon excitation, one carrier issubstantially confined to the core and the other carrier issubstantially confined to the overcoating, and the plurality ofnanocrystals is monodisperse. In another aspect, a coated nanocrystalincludes a core nanocrystal including a first semiconductor material, afirst overcoating including a second semiconductor material on the corenanocrystal, and a second overcoating including a third semiconductormaterial on the first overcoating.

[0010] The conduction band of the first semiconductor material can be athigher energy than the conduction band of the second semiconductormaterial, and the valence band of the first semiconductor material canbe at higher energy than the valence band of the second semiconductormaterial. The conduction band of the first semiconductor material can beat lower energy than the conduction band of the second semiconductormaterial, and the valence band of the first semiconductor material canbe at lower energy than the valence band of the second semiconductormaterial. The nanocrystal can include an organic layer on a surface ofthe coated nanocrystal.

[0011] The method can include exposing the nanocrystal to an organiccompound having affinity for a surface of the coated nanocrystal. Thecoated nanocrystal can be dispersible in a liquid. The firstsemiconductor material can be a Group II-VI compound, a Group II-Vcompound, a Group III-VI compound, a Group III-V compound, a Group IV-VIcompound, a Group I-III-VI compound, a Group II-IV-VI compound, or aGroup II-IV-V compound. The first semiconductor material can be ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS,PbSe, PbTe, or mixtures thereof. The second semiconductor material canbe a Group II-VI compound, a Group II-V compound, a Group III-VIcompound, a Group III-V compound, a Group IV-VI compound, a GroupI-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-Vcompound. The second semiconductor material can be ZnO, ZnS, ZnSe, ZnTe,CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN,AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP,TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof. The firstsemiconductor material can be CdTe and the second semiconductor materialCdSe. The first semiconductor material can be CdSe and the secondsemiconductor material ZnTe. The method can include overcoating a thirdsemiconductor material on the second semiconductor material.

[0012] The third semiconductor material can have a mismatched bandoffset compared to the second semiconductor material. The thirdsemiconductor material can be a Group II-VI compound, a Group II-Vcompound, a Group III-VI compound, a Group III-V compound, a Group IV-VIcompound, a Group I-III-VI compound, a Group II-IV-VI compound, or aGroup II-IV-V compound. The third semiconductor material can be ZnO,ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS,HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs,InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof.

[0013] The nanocrystal can emit light upon excitation, wherein thewavelength of maximum emission intensity is longer than 700 nm, orbetween 700 nm and 1500 nm. The nanocrystal can have a quantumefficiency greater than 10%.

[0014] Other features, objects, and advantages of the invention will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a drawing depicting a semiconductor nanocrystal having atype II heterostructure.

[0016]FIG. 2 is a drawing depicting potential diagrams and theoreticallymodeled electron (dotted line) and hole (solid line) radialwavefunctions for semiconductor nanocrystal heterostructures.

[0017]FIG. 3 are graphs depicting absorptivity and normalizedphotoluminescence spectra of nanocrystals.

[0018]FIG. 4A are bright field transmission electron microscopy (TEM)images of nanocrystals.

[0019]FIGS. 4B and 4C are graphs depicting size distributions of thenanocrystals.

[0020]FIG. 5A is a graph depicting normalized photoluminescence spectraof a monodisperse population of different dimension CdTe/CdSe(core/shell) nanocrystals.

[0021]FIG. 5B is a graph of normalized photoluminescence intensitydecays of a monodisperse population of CdTe/CdSe (core/shell)nanocrystals and a monodisperse population of CdTe nanocrystals.

DETAILED DESCRIPTION

[0022] Colloidal synthesis of nanocrystals can be used to manufacturenanocrystals that have core and overcoating (core-shell) structures andhave type II heterostructures. Colloidal synthesis is described, forexample, in Murray, C. B., et al., J. Am. Chem. Soc. 1993, 115, 8706,Peng, X., et al., J. Am. Chem. Soc. 1997, 119, 7019, Dabbousi, B. O., etal., J. Phys. Chem. B 1997, 101, 9463, and Cao, Y. W. and Banin, U.,Angew. Chem. Int. Edit. 1999, 38, 3692, each of which is incorporated byreference in its entirety. The colloidal synthetic route can be appliedto any II-VI and III-V semiconductor materials.

[0023] Examples of semiconductor nanocrystals having type II structuresare shown in FIG. 1. Upon excitation of a nanocrystal having a type IIheterostructure, the carriers are not confined in one semiconductormaterial. A nanocrystal having a type II heterostructure includes a corenanocrystal of one semiconductor surrounded by a shell of a secondsemiconductor. One carrier can become confined to the core, while theother is mostly confined to the shell. As illustrated in FIG. 1, therelative location of the carriers can depend on the materials in thecore and the shell. The band offsets of the two semiconductor materialscan make spatial separation of the hole and the electron energeticallyfavorable. Nanocrystals having type II heterostructures can have novelproperties because of the spatial separation of carriers, which does notoccur in nanocrystals having type I heterostructures. Nanocrystalshaving type II structures can access emission wavelengths (i.e.wavelengths of maximum emission intensity) that would not otherwise beavailable with the semiconductors making up the nanocrystal.

[0024] GaSb/GaAs and Ge/Si type II nanocrystals grown by molecular beamepitaxy (MBE) have been studied. See, for example, Suzuki, K., et al.,J. Appl. Phys. 1999, 85, 8349, and Schittenhelm, P., et al., Appl. Phys.Lett. 1995, 67, 1292, each of which is incorporated by reference in itsentirety. However, studies of such type II nanocrystals are ratherscarce presumably due to the synthetic difficulties. Colloidal synthesiscan allow easy and flexible size-tuning of nanocrystals andadvantageously provides a simple route to type II structures: CdTe/CdSe(core/shell) and CdSe/ZnTe (core/shell) type II nanocrystals. Oneadvantage of colloidal synthesis is that there is no interference fromwetting layers which is commonly observed in MBE-grown type IInanocrystals.

[0025]FIG. 2 shows the potential energy diagrams of CdTe/CdSe(core/shell) (left side) and CdSe/ZnTe (core/shell) (right side)nanocrystals with theoretically modeled radial wavefunctions for thelowest energy electron (dotted line) and hole (solid line) wavefunctions. The potentials are referenced to vacuum level and are not toscale. For the theoretical modeling, bulk material parameters were usedwith the electron and the hole wavefuctions which satisfy both thewavefunctions and the probability current continuities. See, forexample, Haus, J. W., et al., Phys. Rev. B 1993, 47, 1359, Schooss, D.,et al., Phys. Rev. B 1994, 49, 49, and Yakimov, A. I., et al., Phys.Rev. B 2001, 63, 045312, each of which is incorporated by reference inits entirety. A 20 Å core radius and 4 Å shell thickness was used forthe modeling of both CdTe/CdSe (core/shell) and CdSe/ZnTe (core/shell)nanocrystals. In CdTe/CdSe (core/shell) nanocrystals, the counductionband of the shell is intermediate in energy to the valence band andconduction band of the core. CdTe/CdSe (core/shell) nanocrystals havelower potentials for the holes in the core and for the electrons in theshell. As a result, the holes can be mostly confined to the CdTe core,while the electrons can be mostly confined to the CdSe shell. Themodeled wavefunctions of a CdTe/CdSe (core/shell) nanocrystal (FIG. 2),show that the hole wavefunction resides mostly in the core, while theelectron wavefunction resides mostly in the shell. In contrast,CdSe/ZnTe (core/shell) nanocrystals have the opposite band offsetalignment, with the valence band of the shell intermediate in energy tothe valence band and conduction band of the core. As a result, theelectrons reside mostly in the CdSe cores, while the holes reside mostlyin the ZnTe shells. See FIG. 2.

[0026] CdTe and CdSe cores can be prepared by the pyrolysis oforganometallic precursors in hot coordinating agents. See, for example,Murray, C. B., et al., J. Am. Chem. Soc. 1993, 115, 8706, and Mikulec,F., Ph.D. Thesis, MIT, Cambridge, 1999, each of which is incorporated byreference in its entirety. Growth of CdSe and ZnTe shell layers on thebare CdTe and CdSe cores can be carried out by simple modifications ofconventional overcoating procedures. See, for example, Peng, X., et al.,J. Am. Chem. Soc. 1997, 119, 7019, Dabbousi, B. O., et al., J. Phys.Chem. B 1997, 101, 9463, and Cao, Y. W. and Banin U. Angew. Chem. Int.Edit. 1999, 38, 3692, each of which is incorporated by reference in itsentirety. For example, dimethylcadmium and bis(trimethylsilyl)selenidecan be used for CdSe shell growth. A typical protocol for CdSe shellgrowth for CdTe/CdSe(core/shell) nanocrystals can include dispersingprecipitated CdTe nanocrystals in trioctylphosphine oxide (TOPO), whichhas been dried under vacuum at 160° C., preparing an overcoating stocksolution by mixing of dimethylcadmium and bis(trimethylsilyl)selenide ina 1:1 stoichiometry in trioctylphosphine (TOP), and adding theovercoating stock solution to the CdTe nanocrystal-TOPO mixture at atemperature of between 80° C. and 180° C., depending on the size of CdTenanocrystals used. The mixture can be maintained at this temperatureuntil small nanocrystals (as monitored by optical spectroscopy) of theovercoating form in the mixture, for example three hours. The mixturecan be brought to a higher temperature, e.g. 200° C., and held there fora period of hours to days, until the overcoating is formed. A highertemperature can be necessary for larger size nanocrystals. ZnTe shellgrowth can be obtained by a similar method using diethylzinc andtrioctylphosphine telluride precursors. The cores having a desiredaverage diameter can be obtained from the initial core preparations, andthe thickness of the shell layers can be controlled by the amount of theprecursors used at the subsequent growth step.

[0027] Spatial separation of the carriers in type II nanocrystalssuggests poor overlap between the electron and hole wavefunctions,preventing type II nanocrystals from reaching high emission quantumefficiencies. In the case of CdTe/CdSe (core/shell) nanocrystals,quantum efficiency of up to 4% was obtained. CdSe/ZnTe (core/shell)nanocrystals have quantum efficiency of less than 1%. The lower quantumefficiency of CdSe/ZnTe (core/shell) can be due to smaller overlapbetween the electron and hole wavefunctions than the overlap in ofCdTe/CdSe (core/shell) nanocrystals. Holes confined in the shells, as inthe case of CdSe/ZnTe (core/shell) nanocrystals, have a larger effectivemass than electrons and therefore tend to spread less into the cores.See FIG. 2. Moreover, a system with the holes confined in the shellslike CdSe/ZnTe (core/shell) nanocrystals may be more susceptible to trapsites.

[0028] The photoluminescence emissions from type II heterostructurenanocrystals, such as CdTe/CdSe (core/shell) and CdSe/ZnTe (core/shell)nanocrystals, can occur at longer wavelengths than emmission from thecorresponding cores. The emission from nanocrystals having type IIstructures originates from the radiative recombination of the carriersacross the core-shell interface. Therefore, the energy of the emissiondepends on the band offsets, which are controlled by the coresemiconductor material and size and the overcoating semiconductormaterial and thickness. Type II nanocrystals can have a smallereffective bandgap, and therefore emit at longer wavelengths, thannanocrystals composed of either the core or shell material in the typeII nanocrystal. The nanocrystals having type II structures can beconsidered to have quasi-indirect, or virtual, bandgaps whose energiesare determined by the band offsets. The effective bandgap can beengineered in such structures, since many possible combinations ofsemiconductor materials can be used to tailor the virtual bandgaps.

[0029] To overcome the limitation of quantum efficiency in nanocrystalshaving type II structures, additional layers of semiconductor materialscan be added to the particles. For example, a second coating of ZnTe canbe added to a CdTe/CdSe (core/shell) nanocrystal, and increase thequantum efficiency to as high as 20%. The overcoating procedure can besimilar to the CdSe/ZnTe (core/shell) nanocrystal shell preparation. Theenhanced quantum efficiency is thought to originate from the increasedcarrier wavefunction overlap induced by the mismatched bandgaps. Forincreased quantum efficiency, it can be important for the first coatingand second coating to have a mismatched band offset. For example, when asecond coating of ZnS was deposited on CdTe/CdSe (core/shell)nanocrystals, the emission was red-shifted, but the quantum efficiencydecreased. Although ZnS layers can help passivate the CdSe shell layersurface, the effect was overwhelmed by the decreased wavefunctionoverlaps due to the electron wavefunction leakage into the ZnS layers.

[0030] The nanocrystal can be a member of a population of nanocrystalshaving a narrow size distribution. The nanocrystal can be a sphere, rod,disk, or other shape. The nanocrystal can include a core of asemiconductor material. The nanocrystal can include a core having theformula MX, where M is cadmium, zinc, magnesium, mercury, aluminum,gallium, indium, thallium, or mixtures thereof, and X is oxygen, sulfur,selenium, tellurium, nitrogen, phosphorus, arsenic, antimony, ormixtures thereof.

[0031] The semiconductor forming the core of the nanocrystal can includeGroup II-VI compounds, Group II-V compounds, Group III-VI compounds,Group III-V compounds, Group IV-VI compounds, Group I-III-VI compounds,Group II-IV-VI compounds, and Group II-IV-V compounds, for example, ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS,PbSe, PbTe, or mixtures thereof.

[0032] The core can have an overcoating on a surface of the core. Theovercoating can be a semiconductor material having a compositiondifferent from the composition of the core which is selected to providea type II heterostructure. The overcoat of a semiconductor material on asurface of the nanocrystal can include a Group II-VI compounds, GroupII-V compounds, Group III-VI compounds, Group III-V compounds, GroupIV-VI compounds, Group I-III-VI compounds, Group II-IV-VI compounds, andGroup II-IV-V compounds, for example, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe,HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN,InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixturesthereof. For example, ZnS, ZnSe or CdS overcoatings can be grown on CdSeor CdTe nanocrystals.

[0033] The emission from the nanocrystal can be a narrow Gaussianemission band that can be tuned through the complete wavelength range ofthe ultraviolet, visible, or infrared regions of the spectrum by varyingthe size of the nanocrystal, the composition of the nanocrystal, orboth. For example, CdSe can be tuned in the visible region and InAs canbe tuned in the infrared region.

[0034] The population of nanocrystals can have a narrow sizedistribution. The population can be monodisperse and can exhibit lessthan a 15% rms deviation in diameter of the nanocrystals, preferablyless than 10%, more preferably less than 5%. Spectral emissions in anarrow range of between 10 and 150 nm full width at half max (FWHM) canbe observed. Semiconductor nanocrystals having type II structures canhave emission quantum efficiencies of greater than 2%, 5%, 10%, 20%,40%, 60%, 70%, or 80%.

[0035] The absorption spectra of nanocrystals having type IIheterostructures have characteristic features of smoothly increasingabsorptions to the shorter wavelength region and long tails to thelonger wavelength regions. This can be because the nanocrystals havingtype II structures have indirect band characteristics with spatiallyindirect excitons. Nanocrystals having type II structures can haverelatively weak oscillator strength since the oscillator strengths arestrongly governed by the carrier wavefunction overlaps. See, forexample, Laheld, U .E. H., et al., Phys. Rev. B 1995, 52:2697, andRorrison, J. M., Phys. Rev. B 1993, 48:4643, each of which isincorporated by reference in its entirety. Changes of the nanocrystaltotal oscillator strength before and after the shell growth can becompared because they are proportional to the spectrally integratedabsorptivities of the nanocrystals. The nanocrystals having type IIheterostructures can have near-infrared (near-IR) emissions, forexample, in CdTe/CdSe (core/shell) and CdSe/ZnTe (core/shell)nanocrystals. These nanocrystals can be used as IR chromophores that areresistant against photo-bleaching and various chemical environments. Innanocrystals having type II heterostructures, both the cores and theshells have high probability densities for a carrier. As a result,thickness of the shell, as well as the core diameter, controls theemission energies due to the quantum confinement effect. By selectingdifferent core diameters and shell thicknesses, the emission from typeII nanocrystals can be easily tuned, for example, from 300 nm to 3microns, or 700 nm to over 1000 nm. Nanocrystals having type IIheterostructures can have longer exciton decay times than nanocrystalshaving type I heterostructures, for example, because spatially indirectexcitons decay with much larger time constants.

[0036] Methods of preparing semiconductor nanocrystals include pyrolysisof organometallic reagents, such as dimethyl cadmium, injected into ahot coordinating agent. This permits discrete nucleation and results inthe controlled growth of macroscopic quantities of nanocrystals.Preparation and manipulation of nanocrystals are described, for example,in U.S. Pat. No. 6,322,901, which is incorporated by reference in itsentirety. The method of manufacturing a nanocrystal is a colloidalgrowth process and can produce a monodisperse particle population.Colloidal growth occurs by rapidly injecting an M donor and an X donorinto a hot coordinating agent. The injection produces a nucleus that canbe grown in a controlled manner to form a nanocrystal. The reactionmixture can be gently heated to grow and anneal the nanocrystal. Boththe average size and the size distribution of the nanocrystals in asample are dependent on the growth temperature. The growth temperaturenecessary to maintain steady growth increases with increasing averagecrystal size. The nanocrystal is a member of a population ofnanocrystals. As a result of the discrete nucleation and controlledgrowth, the population of nanocrystals obtained has a narrow,monodisperse distribution of diameters. The monodisperse distribution ofdiameters can also be referred to as a size. The process of controlledgrowth and annealing of the nanocrystals in the coordinating agent thatfollows nucleation can also result in uniform surface derivatization andregular core structures. As the size distribution sharpens, thetemperature can be raised to maintain steady growth. By adding more Mdonor or X donor, the growth period can be shortened.

[0037] An overcoating process is described, for example, in U.S. Pat.No. 6,322,901, which is incorporated by reference in its entirety. Byadjusting the temperature of the reaction mixture during overcoating andmonitoring the absorption spectrum of the core, overcoated materialshaving high emission quantum efficiencies and narrow size distributionscan be obtained.

[0038] The M donor can be an inorganic compound, an organometalliccompound, or elemental metal. The inorganic compound can be a salt. Thesalt can be combined with a coordinating agent, such as an amine. See,for example, U.S. Pat. No. 6,576,291, which is incorporated by referencein its entirety. M is cadmium, zinc, magnesium, mercury, aluminum,gallium, indium or thallium. The X donor is a compound capable ofreacting with the M donor to form a material with the general formulaMX. Typically, the X donor is a chalcogenide donor or a pnictide donor,such as a phosphine chalcogenide, a bis(silyl) chalcogenide, dioxygen,an ammonium salt, or a tris(silyl) pnictide. Suitable X donors includedioxygen, bis(trimethylsilyl) selenide ((TMS)₂Se), trialkyl phosphineselenides such as (tri-n-octylphosphine)selenide (TOPSe) or(tri-n-butylphosphine)selenide (TBPSe), trialkyl phosphine telluridessuch as (tri-n-octylphosphine)telluride (TOPTe) orhexapropylphosphorustriamide telluride (HPPTTe),bis(trimethylsilyl)telluride ((TMS)₂Te), bis(trimethylsilyl)sulfide((TMS)₂S), a trialkyl phosphine sulfide such as(tri-n-octylphosphine)sulfide (TOPS), an ammonium salt such as anammonium halide (e.g., NH₄Cl), tris(trimethylsilyl) phosphide ((TMS)₃P),tris(trimethylsilyl)arsenide ((TMS)₃As), ortris(trimethylsilyl)antimonide ((TMS)₃Sb). In certain embodiments, the Mdonor and the X donor can be moieties within the same molecule.

[0039] A coordinating agent can help control the growth of thenanocrystal. The coordinating agent is a compound having a donor lonepair that, for example, has a lone electron pair available to coordinateto a surface of the growing nanocrystal. The coordinating agent can be asolvent. Solvent coordination can stabilize the growing nanocrystal.Typical coordinating agents include alkyl phosphines, alkyl phosphineoxides, alkyl phosphonic acids, or alkyl phosphinic acids, however,other coordinating agents, such as pyridines, furans, and amines mayalso be suitable for the nanocrystal production. Examples of suitablecoordinating agents include pyridine, tri-n-octyl phosphine (TOP) andtri-n-octyl phosphine oxide (TOPO). Technical grade TOPO can be used.

[0040] Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a sharpparticle size distribution during growth. Reactants can be added to thenucleation solution during crystal growth to grow larger crystals. Bystopping growth at a particular nanocrystal average diameter, apopulation having an average nanocrystal diameter of less than 150 Å canbe obtained. A population of nanocrystals can have an average diameterof 15 Å to 125 Å.

[0041] The particle size distribution can be further refined by sizeselective precipitation with a poor solvent for the nanocrystals, suchas methanol/butanol as described in U.S. Pat. No. 6,322,901,incorporated herein by reference in its entirety. For example,nanocrystals can be dispersed in a solution of 10% butanol in hexane.Methanol can be added dropwise to this stirring solution untilopalescence persists. Separation of supernatant and flocculate bycentrifugation produces a precipitate enriched with the largestcrystallites in the sample. This procedure can be repeated until nofurther sharpening of the optical absorption spectrum is noted.Size-selective precipitation can be carried out in a variety ofsolvent/nonsolvent pairs, including pyridine/hexane andchloroform/methanol. The size-selected nanocrystal population can haveno more than a 15% rms deviation from mean diameter, preferably 10% rmsdeviation or less, and more preferably 5% rms deviation or less.

[0042] The outer surface of the nanocrystal can include a layer ofcompounds derived from the coordinating agent used during the growthprocess. The surface can be modified by repeated exposure to an excessof a competing coordinating group to form an overlayer. For example, adispersion of the capped nanocrystal can be treated with a coordinatingorganic compound, such as pyridine, to produce crystallites whichdisperse readily in pyridine, methanol, and aromatics but no longerdisperse in aliphatic solvents. Such a surface exchange process can becarried out with any compound capable of coordinating to or bonding withthe outer surface of the nanocrystal, including, for example,phosphines, thiols, amines and phosphates. The nanocrystal can beexposed to short chain polymers which exhibit an affinity for thesurface and which terminate in a moiety having an affinity for asuspension or dispersion medium. Such affinity improves the stability ofthe suspension and discourages flocculation of the nanocrystal.

[0043] Transmission electron microscopy (TEM) can provide informationabout the size, shape, and distribution of the nanocrystal population.Powder X-ray diffraction (XRD) patterns can provided the most completeinformation regarding the type and quality of the crystal structure ofthe nanocrystals. Estimates of size are also possible since particlediameter is inversely related, via the X-ray coherence length, to thepeak width. For example, the diameter of the nanocrystal can be measureddirectly by transmission electron microscopy or estimated from X-raydiffraction data using, for example, the Scherrer equation. It also canbe estimated from the UV/Vis absorption spectrum.

EXAMPLES

[0044] All the procedures described here were carried out under an inertatmosphere unless specified otherwise. CdTe and CdSe nanocrystals weresynthesized via pyrolysis of organometallic precursors using previouslydescribed procedures. See, for example, Murray, et al., J. Am. Chem.Soc. 1993, 115, 8706, and Mikulec, F. Ph.D. Thesis, MIT, 1999.

[0045] Preparation of Nanocrystals

[0046] Bis-(trimethylsilyl)selenide was prepared as follows. To 200 mLof a solution of superhydride (Aldrich, 1.0 M solution of lithiumtriethylborohydride in tetrahydrofuran), 7.89 g of Se shot (Strem,99.99%) was added and stirred at room temperature for 2 hours.Chlorotrimethylsilane (25.2 g, Aldrich, 99%) was introduced and furtherstirred for a few hours. Solvents were removed in vacuo. The product wasobtained by vacuum distillation.

[0047] A population of CdTe/CdSe (core/shell) nanocrystals was preparedas follows. Precipitated CdTe nanocrystals were dispersed intrioctylphosphine oxide (TOPO, Alfa, 95%), and dried under vacuum at160° C. An overcoating stock solution was prepared by mixing desiredamounts of 1:1 molar stoichiometry of dimethylcadmium (Strem, 97%) andbis(trimethylsilyl)selenide in trioctylphosphine (TOP, Fluka, 90%).While the CdTe-TOPO mixture was vigorously stirred under Ar flow,prepared overcoating stock solution was added slowly dropwise.Temperature was set at a point ranging from 130° C. to 180° C. dependingon the size of CdTe nanocrystals used. A higher temperature wasnecessary for larger size CdTe nanocrystals.

[0048] Another population of CdTe/CdSe (core/shell) nanocrystals wasprepared as follows. Precipitated CdTe nanocrystals were dispersed in amixture of TOP and TOPO. The mixture was vigorously stirred and heatedto 100° C., and overcoating stock solution prepared as above was thenadded slowly dropwise. The reaction mixture was stirred for 3 hours at100° C. The formation of small CdSe nanocrystals (informally called“magic size” nanocrystals, with a peak absorbance at ˜410 nm) wasmonitored by optical spectroscopy. The reaction temperature was raisedto 200° C. and stirred until the CdSe shell growth is completed. Thesmall “magic size” CdSe nanocrystals fused onto the CdTe nanocrystalcore surface, forming a CdSe shell. The CdSe shell formation step tookup to a few days.

[0049] A population of CdSe/ZnTe (core/shell) nanocrystals generally wasprepared by the method described for CdTe/CdSe nanocrystals. Diethylzinc(Strem, 99.99%) and TOPTe were used as the precursors. TOPTe was made bydissolving Te powder (Strem, 99.999%) in TOP at room temperature.

[0050] Characterization of Nanocrystals

[0051]FIG. 3 shows the absorption and emission spectra of core CdTe andCdSe nanocrystals and corresponding CdTe/CdSe (core/shell) and CdSe/ZnTe(core/shell) nanocrystals having a type II structure. The left graphshows spectra of 32 Å radius CdTe nanocrystals (dashed) and CdTe/CdSe(32 Å radius core/11 Å shell thickness) nanocrystals. The right graphshows spectra of 22 Å radius CdSe nanocrystals (dashed) and CdSe/ZnTe(22 Å radius core/18 Å shell thickness) nanocrystals. Thephotoluminescence spectra were obtained by exciting the nanocrystals at533 nm. Changes of the nanocrystal total oscillator strength before andafter the shell growth can be compared because they are proportional tothe spectrally integrated absorptivities of the nanocrystals. The typeII emission is not related to the deep trap luminescence of the corenanocrystals. In FIG. 3, deep trap luminescence of bare CdSenanocrystals can be found around 750 nm, which does not match theemission wavelength of the corresponding CdSe/ZnTe (core/shell)nanocrystals having type II structures. This excludes the possibilitythat the deposition of shell layers merely quenches the band-edgeluminescence of the cores while the deep trap luminescence remains.

[0052] Actual particle size changes can be seen in the transmissionelectron microscopy (TEM) images of FIG. 4A. FIG. 4A generally showssamples that were used to obtain the spectra of FIG. 3, namely amonodisperse population of 32 Å radius CdTe nanocrystals, a monodispersepopulation of CdTe/CdSe (32 Å radius core/11 Å shell thickness)nanocrystals, a monodisperse population of 22 Å radius CdSenanocrystals, and a monodisperse population of CdSe/ZnTe (22 Å radiuscore/18 Å shell thickness) nanocrystals. The images were obtained usinga JEOL2000 at an operation voltage of 200 kV. FIGS. 4B and 4C are graphsdepicting size distributions of the nanocrystals. The total oscillatorstrengths of the nanocrystals did not change much as a result of shellgrowth despite of the significant increase in actual particle size.Increased number of crystalline unit cells in nanocrystals having typeII structures compared to the corresponding core nanocrystals can beexpected to result in much higher total oscillator strengths, but theeffect is offset by the poor overlap between the electron and holewavefunctions which is induced by the type II electronic structures.This effect is a characteristic feature of nanocrystals having type IIstructures that generally is not found in alloyed systems.

[0053] With the two variables of the shell thickness and the core size,emissions of type II nanocrystals can be easily tuned. As seen in FIG.5A, the emission of CdTe/CdSe (core/shell) nanocrystals can span from700 nm to over 1000 nm simply by changing the core size and shellthickness. The CdTe core radii and CdSe shell thicknesses were 16 Å/19Å, 16 Å/32 Å, 32 Å/11 Å, 32 Å/24 Å, and 56 Å/19 Å from left to right inthe graph. The size of the core and shell thickness was measured via TEMand elemental analysis by wavelength dispersion spectroscopy (WDS). FIG.5B shows room temperature PL decays of CdTe/CdSe (32 Å radius core/11 Åshell thickness ) nanocrystals (solid line) and 32 Å radius CdTenanocrystals (dotted line). The data were obtained by exciting thesamples with a 400 nm pulse laser at 298 K. The PL decay of CdTe/CdSe(core/shell) nanocrystal was much longer than that of the core CdTenanocrystal. This shows the characteristic long decay of spatiallyindirect excitons from nanocrystals having type II heterostructures.

[0054] Other embodiments are within the scope of the following claims.

What is claimed is:
 1. A method of preparing a coated nanocrystalcomprising: introducing a core nanocrystal including a firstsemiconductor material into an overcoating reaction mixture; andovercoating a second semiconductor material on the core nanocrystal,wherein the first semiconductor material and the second semiconductormaterial are selected so that, upon excitation, one carrier issubstantially confined to the core and the other carrier issubstantially confined to the overcoating.
 2. The method of claim 1,wherein the conduction band of the first semiconductor material is athigher energy than the conduction band of the second semiconductormaterial and the valence band of the first semiconductor material is athigher energy than the valence band of the second semiconductormaterial.
 3. The method of claim 1, wherein the conduction band of thefirst semiconductor material is at lower energy than the conduction bandof the second semiconductor material and the valence band of the firstsemiconductor material is at lower energy than the valence band of thesecond semiconductor material.
 4. The method of claim 1, wherein thenanocrystal further comprises an organic layer on a surface of thecoated nanocrystal.
 5. The method of claim 1, further comprisingexposing the nanocrystal to an organic compound having affinity for asurface of the coated nanocrystal.
 6. The method of claim 1, wherein thecoated nanocrystal is dispersible in a liquid.
 7. The method of claim 1,where in the first semiconductor material is a Group II-VI compound, aGroup II-V compound, a Group III-VI compound, a Group III-V compound, aGroup IV-VI compound, a Group I-III-VI compound, a Group II-IV-VIcompound, or a Group II-IV-V compound.
 8. The method of claim 1, whereinthe first semiconductor material is ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe,HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN,InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixturesthereof.
 9. The method of claim 1, wherein the second semiconductormaterial is a Group II-VI compound, a Group II-V compound, a GroupIII-VI compound, a Group III-V compound, a Group IV-VI compound, a GroupI-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-Vcompound.
 10. The method of claim 1, wherein the second semiconductormaterial is ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe,MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb,InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, ormixtures thereof.
 11. The method of claim 1, wherein the firstsemiconductor material is CdTe and the second semiconductor material isCdSe.
 12. The method of claim 1, wherein the first semiconductormaterial is CdSe and the second semiconductor material is ZnTe.
 13. Themethod of claim 1, further comprising overcoating a third semiconductormaterial on the second semiconductor material.
 14. The method of claim13, wherein the third semiconductor material has a mismatched bandoffset compared to the second semiconductor material.
 15. The method ofclaim 13, wherein the third semiconductor material is a Group II-VIcompound, a Group II-V compound, a Group III-VI compound, a Group III-Vcompound, a Group IV-VI compound, a Group I-III-VI compound, a GroupII-IV-VI compound, or a Group II-IV-V compound.
 16. The method of claim13, wherein the third semiconductor material is ZnO, ZnS, ZnSe, ZnTe,CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN,AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP,TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof.
 17. A coatednanocrystal comprising: a core nanocrystal including a firstsemiconductor material; an overcoating including a second semiconductormaterial on the core nanocrystal; and an organic layer on a surface ofthe coated nanocrystal, wherein the first semiconductor material and thesecond semiconductor material are selected so that, upon excitation, onecarrier is substantially confined to the core and the other carrier issubstantially confined to the overcoating.
 18. The nanocrystal of claim17, wherein the conduction band of the first semiconductor material isat higher energy than the conduction band of the second semiconductormaterial and the valence band of the first semiconductor material is athigher energy than the valence band of the second semiconductormaterial.
 19. The nanocrystal of claim 17, wherein the conduction bandof the first semiconductor material is at lower energy than theconduction band of the second semiconductor material and the valenceband of the first semiconductor material is at lower energy than thevalence band of the second semiconductor material.
 20. The nanocrystalof claim 17, wherein the organic layer is obtained by exposing thenanocrystal to an organic compound having affinity for a surface of thecoated nanocrystal.
 21. The nanocrystal of claim 17, wherein the coatednanocrystal is dispersible.
 22. The nanocrystal of claim 17, where inthe first semiconductor material is a Group II-VI compound, a Group II-Vcompound, a Group III-VI compound, a Group III-V compound, a Group IV-VIcompound, a Group I-III-VI compound, a Group II-IV-VI compound, or aGroup II-IV-V compound.
 23. The nanocrystal of claim 17, wherein thefirst semiconductor material ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP,InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof.24. The nanocrystal of claim 17, wherein the second semiconductormaterial is a Group II-VI compound, a Group II-V compound, a GroupIII-VI compound, a Group III-V compound, a Group IV-VI compound, a GroupI-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-Vcompound.
 25. The nanocrystal of claim 17, wherein the secondsemiconductor material is ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe,MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TiSb, TlSb, PbS,PbSe, PbTe, or mixtures thereof.
 26. The nanocrystal of claim 17,wherein the nanocrystal emits light upon excitation, wherein thewavelength of maximum emission intensity is longer than 700 nm.
 27. Thenanocrystal of claim 17, wherein the nanocrystal emits light uponexcitation, wherein the wavelength of maximum emission intensity isbetween 700 nm and 1500 nm.
 28. A population of coated nanocrystalscomprising: a plurality of coated nanocrystals, each coated nanocrystalincluding a core nanocrystal and an overcoating on each corenanocrystal, wherein each core includes a first semiconductor materialand each overcoating includes a second semiconductor material, theplurality of core nanocrystals forming a population of nanocrystals,wherein the first semiconductor material and the second semiconductormaterial are selected so that, upon excitation, one carrier issubstantially confined to the core and the other carrier issubstantially confined to the overcoating; and the plurality ofnanocrystals is monodisperse.
 29. The population of claim 28, whereinthe conduction band of the first semiconductor material is at higherenergy than the conduction band of the second semiconductor material andthe valence band of the first semiconductor material is at higher energythan the valence band of the second semiconductor material.
 30. Thepopulation of claim 28, wherein the conduction band of the firstsemiconductor material is at lower energy than the conduction band ofthe second semiconductor material and the valence band of the firstsemiconductor material is at lower energy than the valence band of thesecond semiconductor material.
 31. The population of claim 28, furthercomprising an organic layer on a surface of each coated nanocrystal. 32.The population of claim 31, wherein the organic layer is obtained byexposing the population to an organic compound having affinity for asurface of a coated nanocrystal.
 33. The population of claim 28, wherein the first semiconductor material is a Group II-VI compound, a GroupII-V compound, a Group III-VI compound, a Group III-V compound, a GroupIV-VI compound, a Group I-III-VI compound, a Group II-IV-VI compound, ora Group II-IV-V compound.
 34. The population of claim 28, wherein thefirst semiconductor material is ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP,InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof.35. The population of claim 28, wherein the second semiconductormaterial is a Group II-VI compound, a Group II-V compound, a GroupIII-VI compound, a Group III-V compound, a Group IV-VI compound, a GroupI-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-Vcompound.
 36. The population of claim 28, wherein the secondsemiconductor material is ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe,MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS,PbSe, PbTe, or mixtures thereof.
 37. The population of claim 28, whereinthe population emits light upon excitation, wherein the wavelength ofmaximum emission intensity is longer than 700 nm.
 38. The population ofclaim 28, wherein the population emits light upon excitation, whereinthe wavelength of maximum emission intensity is between 700 nm and 1500nm.
 39. A coated nanocrystal comprising: a core nanocrystal including afirst semiconductor material; a first overcoating including a secondsemiconductor material on the core nanocrystal; and a second overcoatingincluding a third semiconductor material on the first overcoating. 40.The nanocrystal of claim 39, wherein the first semiconductor materialand the second semiconductor material are selected so that, uponexcitation, one carrier is substantially confined to the core and theother carrier is substantially confined to the first overcoating. 41.The nanocrystal of claim 39, wherein the conduction band of the firstsemiconductor material is at higher energy than the conduction band ofthe second semiconductor material and the valence band of the firstsemiconductor material is at higher energy than the valence band of thesecond semiconductor material.
 42. The nanocrystal of claim 39, whereinthe conduction band of the first semiconductor material is at lowerenergy than the conduction band of the second semiconductor material andthe valence band of the first semiconductor material is at lower energythan the valence band of the second semiconductor material.
 43. Thenanocrystal of claim 39, wherein the third semiconductor material has amismatched band offset to the second semiconductor material.
 44. Thenanocrystal of claim 39, further comprising an organic layer on asurface of the coated nanocrystal.
 45. The nanocrystal of claim 44,wherein the organic layer is obtained by exposing the nanocrystal to anorganic compound having affinity for a surface of the coatednanocrystal.
 46. The nanocrystal of claim 39, where in the firstsemiconductor material is a Group II-VI compound, a Group II-V compound,a Group III-VI compound, a Group III-V compound, a Group IV-VI compound,a Group I-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-Vcompound.
 47. The nanocrystal of claim 39, wherein the firstsemiconductor material is ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe,HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP, InAs,InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof. 48.The nanocrystal of claim 39, wherein the second semiconductor materialis a Group II-VI compound, a Group II-V compound, a Group III-VIcompound, a Group III-V compound, a Group IV-VI compound, a GroupI-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-Vcompound.
 49. The nanocrystal of claim 39, wherein the secondsemiconductor material is ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe,MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS,PbSe, PbTe, or mixtures thereof.
 50. The nanocrystal of claim 39,wherein the third semiconductor material is a Group II-VI compound, aGroup II-V compound, a Group III-VI compound, a Group III-V compound, aGroup IV-VI compound, a Group I-III-VI compound, a Group II-IV-VIcompound, or a Group II-IV-V compound.
 51. The nanocrystal of claim 39,wherein the third semiconductor material is ZnO, ZnS, ZnSe, ZnTe, CdO,CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP,AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs,TiSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof.
 52. The nanocrystal ofclaim 39, wherein the nanocrystal emits light upon excitation, whereinthe wavelength of maximum emission intensity is longer than 700 nm. 53.The nanocrystal of claim 39, wherein the nanocrystal emits light uponexcitation, wherein the wavelength of maximum emission intensity isbetween 700 nm and 1500 nm.
 54. The nanocrystal of claim 53, wherein thequantum efficiency is greater than 10%.